They can be welded up, but it makes them more difficult to assembly onto the motor. I did this with my set of 180's, but they had slip-on collectors, and there was only one specific way I could put them onto the motor. It can be done. I chose not to use them, not because of that, but because I needed the space immediately behind the engine for a number of components (tank, clutch slave cylinder, etc). They are a little different in that I had to make them flatter so that they fit under the Can Am style bodywork. After I made these, I made a set of 4-to-1 that opened up the middle area behind the motor for all of that stuff.